During processing of silver halide photographic elements, the developed silver is oxidized to a silver salt by a suitable bleaching agent. The oxidized silver is then removed from the element in a fixing step.
The most common bleaching solutions contain complexes of ferric ion and various organic ligands. One primary desire in this industry is to design bleaching compositions which are more compatible with the environment, and thus it is desirable to reduce or avoid the use of ferric complex bleaching agents.
Peracid bleaching solutions, such as those containing peroxide, persulfate, perborate, perphosphate, perhalogen, percarboxylic acid or percarbonate bleaching agents, offer an alternative to the ferric complex bleaching solutions. They are less expensive and present lower chemical and biological demands on the environment since their by-products can be less harmful.
While persulfate bleaching agents have low environmental impact, they have the disadvantage that their bleaching activity is slow and thus require the presence of a bleaching accelerator. The most common bleaching accelerators are thiol compounds that have offensive odors.
Because hydrogen peroxide reacts and decomposes to form water, a peroxide based bleaching solution offers many environmental advantages over persulfate and ferric complex bleaching solutions. As a result, many publications describe peroxide bleaching solutions, including U.S. Pat. No. 4,277,556 (Koboshi et al), U.S. Pat. No. 4,301,236 (Idota et al), U.S. Pat. No. 4,454,224 (Brien et al), U.S. Pat. No. 4,717,649 (Hall et al), and WO-A-92/01972 (published Feb. 6, 1992).
In addition, WO-A-92/07300 (published Apr. 30, 1992) and EP 0 428 101A1 (published May 22, 1991) describe peroxide compositions for bleaching high chloride emulsions. These compositions comprise up to 0.3 mole of chloride ions per liter of solution and have a pH in the range of 5 to 11. These particular bleaching solutions, however, cause vesiculation in the processed element.
WO-A-93/11459 describes peroxide bleaching solutions that include two or more water-soluble sequestering agents for complexing with transition metals. These solutions appear suitable for use with low silver paper materials.
Despite all of the efforts of researchers in the art, no peroxide bleaching composition has been commercialized because of various problems including vesiculation (that is, blistering from evolution of oxygen), poor bleaching efficiency and solution instability.
Improved peroxide bleaching solutions for both low and high chloride emulsions are described in copending and commonly assigned U.S. Ser. No. 08/391,805, filed Feb. 21, 1995 by Haye, O'Toole and Ballou, and U.S. Ser. No. 08/391,993, filed on Feb. 21, 1995 by Haye, Marrese and Bonner. These applications describe the improvements achieved in bleaching efficiency and speed and reduced vesiculation obtained by including at least 0.35 mole of chloride ions per liter of solution.
There remains a need, however, for highly efficient peroxide bleaching solutions which have improved stability.